This invention relates to an improved process for generating an aqueous solution containing at least one active bromine compound which may be used for water treatment and the recovery of precious metals such as gold.
The electrogeneration of bromine is well known. The typical process for electrogenerating bromine is described in European Patent Application Publication No. 476,862, published Mar. 25, 1992, filed in the name of Great Lakes Chemical Corporation. In that process, an aqueous solution containing sodium bromide is electrolyzed at a controlled acidic pH to produce the desired bromine solution. The oxidation of the bromide ion to bromine at acidic pH yields 1.0 g bromine equivalent per 1.0 g bromide. Volatile hydrogen is also formed which must be ventilated by some means. This process for generating bromine is inadequate, however, since the yield of bromine per starting bromide material is low due to the chemistry of the reaction, and some of the bromine is invariably lost during ventilation of the volatile hydrogen gas.
Equivalent bromine in the form of hypobromous acid also has been formed by electrolyzing a solution containing a combination of chloride and bromide ion, as described in European Patent No. 391,924, issued Mar. 25, 1992, to Cogent Limited ("the Cogent Limited EP Patent"). In that process, the optimal yield of hypobromous acid is generated by starting with an initial molar ratio of chloride:bromide of 4:1, and controlling the rate and period of electrolysis. This process is not adequate for generating bromine, however, because bromate, which has no biofouling control capabilities, is invariably formed. The Cogent EP Patent suggests that the formation of unwanted bromate may be minimized by reducing the period of electrolysis.
Aqueous bromine also may be formed from an aqueous mixture of bromide-bromate salts without electrolysis by acid addition as described in U.S. Pat. No. 3,222,276, issued Dec. 7, 1965 to L. E. Belohlav and J. R. Underhill, Lafayette ("the Belohlav U.S. Patent"). Such a process for producing bromine on-site, however, is also not adequate because it is not economical and requires the transportation of hazardous (oxidizer) bromate.
The present invention provides an improved process for generating bromine which (1) substantially eliminates the formation of bromine gas at controlled acidic pH thus minimizing excess bromide consumption and reducing raw material costs; (2) substantially eliminates unwanted bromate contamination in the final bromine solution; and (3) does not require the transportation of hazardous (oxidizer) bromate.